1. Field of the Invention
The instant invention relates to a method and apparatus for applying molten solder to articles. In particular, the invention is directed to the formation of a uniformly overflowing solder wave from a solder pot in which the articles are dipped or immersed.
2. Description of the Prior Art
Manufacturers and users of printed wiring boards (PWB's) have been faced with a concern over the solderability of the electrically conductive surfaces of such boards since the onset of mass circuit fabrication technology. Extensive research has been conducted on the various material combinations applicable to the problem of preserving solderability, including both fusible and soluble metallic coatings and organic protective finishes. Additionally, numerous methods of applying coatings or treating the surfaces have been developed over the years. Nevertheless, no one single coating method appears to answer the needs of the industry. Economic considerations, quality requirements, and technological developments in processing materials and techniques have a strong influence on the choice of solderability protection method.
Electroplating a 200 to 300 micro-inch coating of 60/40 tin/lead solder alloy has been the most commonly used method of preserving solderability of the electrically conductive surfaces, for the coating also serves as an etch resist in subtractive circuit fabrication. However, with the advent of additive circuit fabrication techniques, most circuit paths are isolated from a common conductive connection and electroplated solder is not a suitable choice for a coating. A recently developed solder leveling process using high pressure gas jets, has been combined with the application of solder to copper circuit paths by dipping the PWB into molten solder to coat the copper for purposes of preserving solderability. U.S. Pat. No. 3,865,298 which issued to Allen et al. on Feb. 11, 1975 describes an apparatus to implement such a gas leveling process.
The Allen apparatus provides a static, heated, solder pot having a rectangular cross section. A pair of parallel slotted rails extend vertically upward from the solder pot and a pair of air nozzles or knives are disposed above the pot on opposite sides thereof. In operation, the PWB is fluxed, inserted in the slotted rails and urged downward into the solder pot to apply molten solder thereto. The PWB is then removed from the pot by pulling the board upward along the rails while simultaneously directed a gas from the jets at the board to remove excess solder therefrom leaving a uniformly thin protective coating of solder on the copper.
However, in such a static solder pot dross and flux residues accumulate on the top of the body of molten solder and tend to deposit or cling to the PWB as it is being removed from the solder causing an undesirable residue coating on the PWB. One solution to this problem is to place the solder pot within a larger tank of molten solder with the top of the solder pot above the level of the solder in the larger tank. Solder is then pumped from the larger tank directly through a wall of the solder pot to cause the solder pot to continuously overflow and remove most of the dross and flux residues formed on the surface thereof.
Although this solution is a substantial improvement over the static solder pot, problems still arise. The flow of the molten solder into the solder pot causes the shape of the wave of solder overflowing from the pot to be non-uniform, resulting in depressions or low points in the overflowing wave. The dross and flux residues tend to accumulate in such depressions and, undesirably, deposit on the surface of the PWB as it is being removed from the pot resulting in the aforementioned undesirable residue coating.
Accordingly, there is a need for a solder pot for dip or immersion soldering in which a uniform overflowing wave can be formed to continuously remove dross and flux residues from the surface of the molten solder.